Diagnosis of Pityriasis Rosea
Usually, a doctor may make a diagnosis of pityriasis rosea solely on the basis of its appearance, particularly the onset of the distinct large herald patch and the symmetrical Christmas tree presentation. Also, the herald patch tends to have a fine scale with a definite border, the so-called "collarette." To rule out other types of skin disorders, a physician may scrape the skin and examine the scales under the microscope to detect fungus infection that could mimic pityriasis rosea. Also, blood tests including rapid plasma reagent (RPR) may be done to detect secondary syphilis, which also may mimic pityriasis rosea. In some cases, a skin biopsy may be required to rule out other skin conditions.